Methyl p-vinylbenzoate has been prepared from a number of different synthetic pathways over the years including the most direct route, esterification of p-vinylbenzoic acid itself. However, even with the direct esterification route itself, one must first prepare the p-vinylbenzoic acid which can involve a number of synthetic sequences in itself. Thus, a number of approaches have been developed which produce methyl p-vinylbenzoate without going through the acid first.
The most direct olefination route prior to this invention involved reacting methyl p-formylbenzoate under boron-Wittig like conditions as is described by Saki, et al in Tetrahedron, 52(3), 915, 1996. However, the use of Knochel's borylmethylcopper reagent does not lend itself for large-scale industrial reactions nor is it economical for producing such quantities.
Several olefination routes based on Heck and Heck-related arylation of ethylene have been reported. One such method involving the palladium catalyzed arylation of ethylene with methyl p-bromobenzoate has been described by J. Kiji, et al in J. Mol. Cat. A: Chem. 97, 73, 1995. A reaction involving methyl p-iodobenzoate has been described by Rule and Fugate in U.S. Pat. No. 4,935,559 (assigned to Eastman Kodak). Both of the latter reactions involve the use of halogenated compounds which one must address environmental concerns. Other related chemistries involve methyl p-chlorosulfonylbenzoate as described by Kasahara, et al in Chem. Ind., 6, 192, 1989 and arenediazonium salts as described by Kikukawa, et al in Bull. Chem. Soc. Jpn., 52(9), 2609, 1979.
Still, other routes involve the oxidation of methyl p-ethylbenzoate to either methyl p-acetylbenzoate first as in British patent 636,196 (assigned to Monsanto Chemical Company) and by Emerson, et al in J. Am. Chem. Soc., 68, 674, 1946 or to the methyl p-alpha-hydroxyethylbenzoate via the bromo-derivative as described by Bergmann and Blum in J. Org. Chem., 24, 549, 1959. Although these routes could produce large-quantities of methyl p-vinylbenzoate, they too have several synthetic steps not mentioned which adds to the cost of production.
Methyl p-vinylbenzoate has been prepared by several groups of individuals which is more suited to academic and smaller research laboratories. These processes include p-methylacetophenone described by Bergmann and Blum, or through either p-cyanoacetophenone or p-dibromobenzene, both described by Marvel and Overberger in J. Am. Chem. Soc., 67, 2250,1945.
The process of direct olefination of an aromatic aldehyde with ketene in the presence of a potassium salt has been described previously by Hurd and Thomas in J. Am. Chem. Soc., 55, 275, 1933 and by Vittum in his PhD Thesis, Cornell University, 1933. The most detailed work describing this olefination which in many ways resembles the Perkin reaction has been described by Vittum. Most reactions were carried out using benzaldehyde with ketene and some type of "catalyst" to prepare styrene and cinnamic acid, the primary reaction products from this reaction. It has been found that temperature variations over a relatively wide range have little impact on the yields and ratios of styrene and cinnamic acid. Additionally, a salt is needed in the course of the reaction and in particular, potassium salts are preferred.
Prior studies involving substituted aromatic aldehydes such as meta-and para-nitrobenzaldehyde, and anisaldehyde (para-methoxybenzaldehyde) suggest that direct olefination of the aldehyde group with ketene is not very feasible. In these cases, no reaction products of meta- or para-nitrovinylbenzene (meta-or para-nitrostyrene) or para-methoxyvinyl-benzene (para-methoxystyrene) was produced and only in the case of meta-nitrobenzaldehyde were any of the corresponding cinnamic acid derivatives isolated. In particular, the starting aldehyde is generally recovered or a tarry residue formed in addition to the recovered starting aldehyde. Most substituted aldehydes in the Perkin reaction, especially those substituted in the para position have a large negative influence on the rate of reaction. Additionally, the type of substituent can have a large influence on the reactivity of the aldehyde in the Perkin reaction.
Thus, the need exists for a process for preparation of methyl p-vinylbenzoate from the reaction of methyl p-formylbenzoate with a ketene in the presence of a potassium salt. The present invention provides such a process.